Extended Sources

Analysis Guide

Scientific analysis of even X-ray point sources can be a
complicated process. This situation is exacerbated for extended
sources, such as clusters of galaxies or supernova remnants, due
to spatial variations in the detector properties. We loosely
define an extended source as any object larger than several
times the telescope
point spread function
and/or encompassing a
region large enough to exhibit significant variations
in the detector properties. Many of the typical analysis
tasks for extended sources are not required for point
source analysis. In this guide, we provide threads for several
common extended source analysis tasks; examples based on archived
Chandra
ACIS
datasets are used.

Before analyzing any data, make sure that it has been processed with
the latest calibration. There are also some filtering choices that
should be considered. Both of these topics are outlined in the
ACIS Data Preparation
analysis guide.

Determining the background for spectral analyses or measuring surface
brightness profiles can be difficult for extended sources where the
object covers a large fraction of the chip. For datasets which do not
allow a local background to be determined, the ACIS calibration team
had compiled a set of experimental "blank-sky" datasets. These files
can be used to create background spectra for spectral fitting or
images for spatial analyses tailored to a specific observation.
When working with these background files, however, the event file must
be filtered to match how the "blank-sky" files were created.

Observations of extended sources often contain serendipitous point
sources not directly associated with the desired target. Although such
objects may be scientifically interesting in their own right, they can
be a source of complication for the analysis of any diffuse emission
from the intended source. Most users will want to filter
out any bright point sources before determining, for example,
a radial surface brightness profile in order
to avoid erroneous sharp discontinuities, or before
fitting spectral models to avoid biasing the resulting fit towards
an incorrect physical model. Removing point sources from the event
file prior to any of these tasks is straightforward in CIAO using
the dmcopy command; however, the first step is
to identify these objects. The CIAO Detect package contains two
tools which provide the ability to work in complex fields with both
point and extended sources -
vtpdetect and
wavdetect.

Determining the properties of diffuse emission from extended
sources can be complicated by spatial variations in the exposure.
For example, a uniform source of illumination will still show
a radial variation in the number of detected counts due to the
telescope vignetting. Similarly, detector features such
as chip gaps can create erroneous spatial structure in the observed
source. Users may correct for these effects by "flat-fielding"
a given image using an
exposure map
appropriate for the observation,
resulting in a fluxed image [photons cm-2
s-1 pixel-1]
of the source. Integration of the resulting flux image values
in a given region provides the total flux in that region.
The exposure map for a given dataset represents the time-averaged
effective area at given sky position over the course of the
observation.

Since the Chandra effective area is a function of energy as well
as position, determination of accurate flux values will depend
on the spectrum of the source. For sources with relatively
flat spectral energy distributions, an exposure map created at
a single energy (such as the peak of emission) may suffice.
For sources which exhibit significant spectral variation over
the desired bandpass, an energy weighted exposure map may yield
more accurate flux values. In such cases, if the broad features
of the incident spectrum are known (e.g. thermal emission from a
cluster) and if the spectral variations within the image are not
too large, one can reduce the systematic errors by computing an
exposure map which is weighted according to a specific model for the
incident spectrum.

One of the most basic analysis tasks for an extended source is the
calculation of a radial surface brightness profile. For objects
such as clusters of galaxies, this profile is the basic measurement
for determining such quantities as the underlying gas density or
the total mass of the cluster. In this thread, we illustrate how to
calculate a radial profile in CIAO for a series of concentric
elliptical annuli. The resulting surface brightness profile may
then be imported into
Sherpa
and fit to a specified
analytic model, e.g. the standard Beta model distribution often
used to describe cluster profiles.

Several different options are available in CIAO for treating the
CCD response, depending on the type of observation and the science
goals of the analysis. In some cases, a single
RMF
and
ARF pair may be
a reasonable choice. In other instances, a simultaneous fit using
multiple RMFs and ARFs may yield better results.

Once the appropriate response files have been created, it is possible
to load the spatial data into
Sherpa
and fit it. If an
exposure map
has been created for the dataset, it may be input
and used directly as a model component in the fitting.

Creating a "pretty picture" of the diffuse emission for an impending
paper is frequently the final step in dealing with extended sources.
It is possible
to detect and remove any point sources in the emission, filling in the
holes via interpolation of the background. The final product, an
exposure-corrected image, should be used cautiously for scientific
purposes since it is highly processed.